The Internal Heat Exchanger (IHX) has become an increasingly common component of motor vehicle air conditioning systems. The IHX is used to increase an operating efficiency of a standard refrigeration cycle for use in an air conditioning system. A standard refrigeration cycle includes a compressor, a condenser, a thermal expansion device, and an evaporator. The IHX is a liquid-to-vapor heat exchanger having an inner channel disposed within an outer channel. A refrigerant used in the refrigeration cycle exits the condenser as a hot liquid and flows through one of the channels as the same refrigerant exits the evaporator as a cool vapor refrigerant that flows through the other channel. The IHX transfers additional heat from the hot liquid refrigerant to the cool vapor refrigerant, cooling the liquid refrigerant below its condensation temperature, also referred to as “sub-cooling.” This cooling of the liquid refrigerant before it reaches the thermal expansion device causes the IHX to utilize cooling capacity that would otherwise be wasted.
One form of the IHX is a tube-in-tube heat exchanger. The tube-in-tube heat exchanger utilizes a tube arrangement having an inner tube disposed co-axially within an outer tube, the interior surface of the inner tube defining a first flow channel while an exterior surface of the inner tube cooperates with an interior surface of the outer tube to form a second flow channel. The cool vapor refrigerant flows through the first flow channel while the hot liquid refrigerant flows through the second channel. Heat is exchanged between the first flow channel and the second flow channel via the wall of the inner tube, which is heat conductive. The tube-in-tube heat exchanger is advantageous because it requires no moving parts, causing the tube-in-tube heat exchanger to rarely require repair or replacement.
Because the tube-in-tube heat exchanger includes a co-axial tube-in-tube configuration, the capacity of the tube-in-tube heat exchanger to exchange heat between the hot liquid refrigerant and the cool vapor refrigerant is directly affected by a length of the co-axial tubes forming the tube-in-tube heat exchanger. Co-axial tubes having a greater length aid the efficiency and cooling capacity of the IHX because the greater length exposes a greater surface area available for heat exchange between the two flow channels formed by the inner tube and the outer tube. However, space constraints present in a vehicle body housing the tube-in-tube heat exchanger often prevents the tube-in-tube heat exchanger from being formed as a single linear leg of tubing. Instead, it has been found that introducing several bends to the tube-in-tube heat exchanger may aid in resolving size constraints, allowing a shape of the tube-in-tube heat exchanger to be adapted to various configurations of adjacent components present within the vehicle.
When forming a bend in a portion of a tube-in-tube heat exchanger, it is preferable to maintain a relatively constant cross-sectional profile of the inner tube relative to the outer tube. However, the process of bending the tube arrangement often results in a warping of the tubes. Specifically, when bent, such tubes tend to take on an oval shape, with a cross-section of the tubes becoming elongated in a direction parallel to an axis of rotation of the tube arrangement as it is bent. In some cases, the warping of the tubes may lead to a collapse of one of the tubes forming the tube-in-tube heat exchanger. The collapse results in a cross-section of the tube taking on a D-shape, with the flattened portion of the D-shape being formed on an inner surface of the bend formed in the tube arrangement.
The presence of a D-shaped collapse in one or both of the tubes forming the tube-in-tube heat exchanger is problematic for several reasons. First, a collapse of both the inner tube and the outer tube results in both tubes having the generally D-shaped cross-section. The generally flat portions of the D-shaped cross-sections tend to contact each other or be in close proximity due to the deformation of both tubes. Vibrations caused by operation of the motor vehicle may cause these flat portions to rattle against each other, causing undesirable noise to be generated within the tube-in-tube heat exchanger. Second, the undesirable deformation of the inner and outer tubes may cause the first and second flow channels to become obstructed, narrowed, or widened undesirably in certain regions, potentially leading to flow restrictions, pressures losses, or regions of inefficient heat transfer.
One method of avoiding the collapse of the tube arrangement has been to preform the tubes forming the tube arrangement to already have the bends present therein rather than applying a force to bend an already assembled tube arrangement. However, such preforming methods often add excessive cost and complexity to the manufacturing process.
It would therefore be desirable to develop a method of bending a tube arrangement having an inner tube disposed within an outer tube that prevents collapse of the tubes forming the tube arrangement while also minimizing a restriction of the flow channels formed by the inner tube and the outer tube.